A carefully designed epidemiologic follow-up study has been initated to investigate a broad range of important scientific issues in detection, program evaluation, etiology, and natural history of breast cancer. The population of approximately 280,000 women intensively screened over a five year period in the Breast Cancer Detection Demonstration Project (BCDDP) provides a unique base from which groups can be selected to study these issues. Although efficacy cannot be established, follow-up provides the opportunity to evaluate the impact of the screening programs on both the screenee and the health care delivery system with special consideration given to the established goals of the BCDDP. This Contractor is one of 28 contractors participating in this study and will follow approximately 2300 women for five years.